Nervous Tissue

Nervous System Controls and integrates all body activities Basic functions: Sense change Interpret and remember change React to changes

Nervous vs Endocrine System Nervous system electrical fast local Endocrine system chemical slow general

CNS – brain/spinal cord Nervous System CNS – brain/spinal cord Integration Processing input output Sensory Motor stimulus PNS response

Organization Central Nervous System – CNS brain spinal cord Peripheral Nervous System - PNS somatic (SNS) sensory motor autonomic (ANS) motor parasympathetic sympathetic

Neurons Functional unit of the Nervous System Dendrite Cell body Nissl substance Axon hillock Neurofibrils Collateral branch One Schwann cell Node of Ranvier Schwann cells, forming the myelin sheath on axon Nucleus terminal Mitochondrion (a) Transmit electrical impulses (action potentials)

Structural Classes of Neurons

Functional Classes of Neurons Afferent

Functional Classes of Neurons Efferent

Functional Classes of Neurons Interneurons

Functional Classes of Neurons Dendrites Peripheral process (axon) Ganglion Cell body Sensory neuron Central process (axon) Spinal cord (central nervous system) Motor neuron Interneuron (association neuron) Afferent transmission nervous system Receptors To effectors (muscles and glands) Efferent transmission

Neuroglia

Schwann Cells Schwann cell cytoplasm plasma membrane nucleus Axon (b) (a) Neurilemma Myelin sheath (c) Myelin: ‘Insulates’ axon. Increases transmission of signal. Node of Ranvier: Exposed axon between Schwann cells

Gray and White Matter

Overview of Nervous Function Skeletal muscles Brain Right side of brain Left side of brain Cerebral cortex Interneuron Upper motor neuron Thalamus Spinal cord Neuromuscular junction Key: Graded potential Nerve action potential Muscle action potential Sensory receptor neuron Lower motor neuron 1 2 3 8 7 4 6 5

Ion Channels Leakage Channel Ligand-gated channels Mechanically gated channels Voltage-gated channels Ion Channels Animation

(b) Ligand-gated channel Ion Channels Extracellular fluid Plasma membrane Cytosol K+ leak channel closed open K+ Channel randomly opens and closes (a) Leakage channel Extracellular fluid Plasma membrane Cytosol Ligand-gated channel closed Chemical stimulus opens the channel (b) Ligand-gated channel channel open Na+ Ca2+ Acetylcholine K+

Ion Channels Extracellular fluid Plasma membrane Cytosol Mechanically gated channel closed Mechanical stimulus opens the channel (c) Mechanically gated channel Mechanically gated channel open Na+ Ca2+ Extracellular fluid Plasma membrane Cytosol Voltage-gated K+ channel closed Change in membrane potential opens the channel (d) Voltage-gated channel Voltage-gated K+ channel open K+ Voltage = –50 mV Voltage = –70 mV

Ion Channels

Electrical Signals in Neurons Like muscle fibers, neurons are electrically excitable. They communicate with one another using two types of electrical signals: Graded potentials are used for short-distance communication only. Action potentials allow communication over long distances within the body. 20

Resting Membrane Potential Negative ions along inside of cell membrane & positive ions along outside potential energy difference at rest is -70 mV Resting potential exists because concentration of ions different inside & outside extracellular fluid rich in Na+ and Cl- cytosol full of K+, organic phosphate & proteins membrane permeability differs for Na+ and K+ 50-100x’s greater permeability for K+ inward flow of Na+ can’t keep up with outward flow of K+ Na+/K+ pump removes Na+ as fast as it leaks in

Resting Membrane Potential Extracellular fluid Plasma membrane Extracellular fluid (a) Distribution of charges that produce the resting membrane potential of a neuron Cytosol Equal numbers of + and – charges in most of ECF Equal numbers of + and – charges in most of cytosol Resting membrane potential (an electrical potential difference across the plasma membrane)

Resting Membrane Potential

Graded Potential Typically on dendrites or cell body Graded means that potential varies in amplitude. Stronger the stimulus, greater the amplitude. Stronger the stimulus the farther it will travel. Decreases as it gets farther away from the stimulus point. Graded Potentials Animation

Graded Potential

Binding of acetylcholine Graded Potential Extracellular fluid Plasma membrane Cytosol Ligand-gated channel closed (b) Depolarizing graded potential caused by the neurotransmitter acetylcholine, a ligand stimulus Ligand-gated channel open Na+ Binding of acetylcholine Depolarizing graded potential Resting membrane Ca2+ Acetylcholine K+ Extracellular fluid Plasma membrane Cytosol Ligand-gated channel closed (c) Hyperpolarizing graded potential caused by the neurotransmitter glycine, a ligand stimulus Ligand-gated channel open Binding of glycine Hyperpolarizing graded potential Resting membrane Cl– Glycine

Graded Potential

Graded Potential

Action Potential

Action Potential Extracellular fluid Plasma membrane Cytosol Time K+ Inactivation gate open Na+ Na+ channel K+ channel Activation gate closed mV 1. Resting state: All voltage-gated Na+ and K+ channels are closed. Axon plasma membrane is at resting membrane potential: small buildup of negative charges along inside surface of membrane and equal buildup of positive charges along outside surface of membrane.

Action Potential 2. Depolarizing phase: Time K+ Na+ mV 2. Depolarizing phase: When membrane potential of axon reaches threshold, Na+ channel activation gates open. As Na+ ions move through these channels into neuron, buildup of positive charges forms along inside surface of membrane and membrane becomes depolarized.

Action Potential mV Time Na+ 3. Repolarizing phase begins: K+ Na+ mV 3. Repolarizing phase begins: Na+ channel inactivation gates close and K+ channels open. Membrane starts to become repolarized as some K+ ions leave neuron and few negative charges begin to build up along inside surface of membrane.

Action Potential Na+ K+ 4. Repolarization phase continues: Time Na+ mV 4. Repolarization phase continues: K+ outflow continues. As more K+ ions leave neuron, more negative charges build up along inside surface of membrane. K+ outflow eventually restores resting membrane potential. Na+ channel inactivation gates open. Return to resting state occurs when K+ gates close. K+

Action Potential

Comparison of Graded & Action Potentials

Continuous Conduction Cell body Na+ Leading edge of action potential (a) Continuous conduction Current flow due to opening of Na+ channels Trigger zone Time 1 msec 5 10

(b) Saltatory conduction Cell body Na+ Leading edge of action potential (b) Saltatory conduction Current flow due to opening of Na+ channels Trigger zone Time 1 msec 5 10 Nodes of Ranvier

Stimulus Intensity How do we differentiate a light touch from a firmer frequency of impulses firm pressure generates impulses at a higher frequency number of sensory neurons activated firm pressure stimulates more neurons than does a light touch

Signal Transmission at Synapses 2 Types of synapses electrical ionic current spreads to next cell through gap junctions faster, two-way transmission & capable of synchronizing groups of neurons chemical one-way information transfer from a presynaptic neuron to a postsynaptic neuron axodendritic -- from axon to dendrite axosomatic -- from axon to cell body axoaxonic -- from axon to axon

Chemical Synapse Presynaptic neuron Nerve impulse Ca2+ Postsynaptic neuron Nerve impulse 1 Presynaptic neuron Synaptic end bulb Neurotransmitter receptor Ligand-gated channel closed Ca2+ Voltage-gated Ca2+ channel Cytoplasm channel open Na+ Synaptic vesicles Postsynaptic potential Synaptic cleft 2 3 4 5 6 7

Neurotransmitters ATP and Other Purines Acetylcholine Nitric oxide Neuropeptides endorphins enkephalin dynorphins substance P Acetylcholine Amino Acids glutamate and aspartate GABA and glycine Biogenic amines norepinephrine epinephrine dopamine serotonin

Neurotransmitters

Postsynaptic potentials Excitatory postsynaptic potential (EPSP) Na+ and K+ gates open at the same time, Na+ diffuses faster results in a depolarizing potential

Postsynaptic Potential Inhibitory postsynaptic potential (IPSP) Membrane made more permeable to K+ and Cl-, Na+ not affected results in a hyperpolarization

Removal of Neurotransmitter Neurotransmitter must be removed from the synapse for normal synaptic function. - Diffusion - Enzymatic degradation - Uptake by cell Events at the Synapse

Summation

Summation Presynaptic neuron 3 Cell body Dendrites Trigger zone (net summation of EPSPs and IPSPs determines whether an action potential is generated here) Excitatory neurotransmitter Postsynaptic neuron Cell body Dendrites Axon terminal Inhibitory Presynaptic neuron 5 Presynaptic neuron 4 Presynaptic neuron 3 Presynaptic neuron 2 Presynaptic neuron 1 EPSP IPSP